I'm itching to push some glass but don't want to commit to any construction projects right now so I'm thinking of getting into making a reference flat. I've read Texereau's three flat method as well as Ed's water test method. Both are well documented, however I have a couple questions about each...

With the water method, a collimated source of light is needed but how can I make this source of collimated light and how accurate does it need to be. I haven't decided on a flat size yet but say I go for 8", if I need to buy a precision lens at least that size to get my collimated light then that could be a real issue. Especially if it needs to be a significant amount more accurate than the flat I want to make.

The Texereau method uses testing one piece next to another - basically comparing errors. I'm sure it works out in the end but it doesn't give me a warm fuzzy having to judge how good my end product is by comparing it to a potentially imperfect reference.

So which is the best way to go to make a flat that I can be confident about it's quality?

You can always search eBay for a comparator mirror which maybe flat to some degree and try to make it flatter. Save all that grinding. I have an 8" that has an edge issue otherwise it would be nice for a ref for double pass tests. Surplus Shed has 3" flat that you can subdia test.

You could make three good primarys in the time of a flat not easy to get it very flat What you plan on needing it for ? Be better to spend time polishing a flat than watching TV I see the fun .. all good I do think it harder than a primary

Check out Advanced Telescope Making Techniques Vol 2. Articles by Browne and Cox show the use of a smaller spherical mirror, (Ritchey-Common) which I have determined works well. Still, I used three 8" disks for the grinding, and a precision straightedge.

As Kfrederich asks, what's it for? For autocollimating, it needs to be smooth, not exactly flat.

A smaller ref flat will be useful in checking edge via fringes. Maybe you can borrow a flat?

Ryan, For the water test the collimator doesn't need to be anything precision, I use the center section of a fresnel lens from an old big screen projection TV and it works just fine. I would avoid too fast an F number lens as many fresnels are however. You could use a plano-convex lens or a telescope mirror as well, SA doesn't matter.

I don't have an immediate need for a flat... was just sort of a make work project and I think it would be handy to have a reference flat from time to time. Eventually though I would like to make my own secondaries which would require one as well.

Ed, thanks for the info. Is it necessary or usefull to use a slanted piece of 50/50 glass so the fringes are viewed 'on axis' similar to other light boxes I've seen people build for interference testing? Also what would you consider to be a cutoff point for f/# if I happen to come across a fresnel?

Ryan, Light boxes with slanted glass don't usually have a collimator which is required for the water test. However if you are making a flat for an autocollimator you can use a light box. Without a collimator a true flat will show some rings of power depending on the water thickness, viewing distance and mirror size. You could use fringe analysis software to correct to absolute flatness but a collimator is easier.

For testing diagonals you wouldn't need so big a collimator. How fast is too fast? Not really sure, I'd like 4:1 but maybe faster would still work?

As I said, you can test with a flat without the colliamitng lens if you observe at a distance equal to at least 10 diameters of the mirror being tested. Thus, for a 10-inch mirror, a distance of 100 inches or 2.5 meters (or 8 feet). To observe the fringes, use a camera with a telephone lens, or a small finder scope.

The collimator only allows you to view the fringes closer, by removing the parallax effect which is made negligible by increasing the viewing distance.

Another thing that makes, making a flat so much easier is a "pressing flat". This is a flat that you use to press your pitch lap against to make it flat. Your trying to make an optically flat surface so if you already have a flat to make your lap, flat the glass will wear much faster to that shape. So you press for 10 minutes and polish for 10 minutes. "Pressing flats" are usually an old flat that the surfaces is scratch up.

I recommend using a round blank then cutting it elliptical if needed. I'm starting to hate working with (Newtonian)ellipses. The sides aren't even vertical. How much is gained using an ellipse than a round one? 3% light loss and contrast?

I recommend using a round blank then cutting it elliptical if needed. I'm starting to hate working with (Newtonian)ellipses. The sides aren't even vertical. How much is gained using an ellipse than a round one? 3% light loss and contrast?

Far better to start elliptical, and block up pieces of 'junk' glass surrounding it, making a rough circle of larger diameter. The buffer thus provided eliminates turned edge. It's rather easier to get good flatness on a *portion* of a larger construct. It might be initially attractive to cut out your piece from a larger single one, but the risk of damage is present, not to mention the possibility of some strain relief and hence warping.

I should have added that if you intend to use the flat for auto-collimation testing then a light box will work just fine. If you view from some distance away the few fringes that may result wouldn't be an issue.

The fringes do wiggle around but it's not a big deal. Take a look at this video of a 4 1/4 diagonal tested on the water test. It doesn't detract from the usefulness of the test.

Ed, I can't tell which is the true shape of the fringes, as they move incessantly. It's not about usefulness, but preference. If I didn't have a reference flat, I would use the Ricthey-Common test with a spherical mirror simply because I prefer a steady image. .

Just like in toolmaking, you make your own tools, and build from first principles. First a sphere..can test it reliably to impeccable quality and then use it to make a larger flat of impeccable quality...to make a paraboloid of impeccable quality, etc. How about a 6 or 8" sphere, f/12-15 of 1/40 wave? If nothing else it will make a fine planetary scope for someone.